For joining bristle bundles to bristle carriers, both of which are made from plastic, numerous processes are known. In conjunction with the present invention only those processes are involved in which the bristle bundle is provided with a thickened portion, inserted in prepared holes on the bristle carrier and connected thereto by the melting or thermoforming of one of the two parts to be joined and the fixed connection is obtained during the solidification or hardening of the melt or the molecular reorientation of the thermoformed area.
In connection with the known welding (e.g. DD-A-221 633), which generally presupposes the same plastic for the bristles and bristle carrier, the ends of the bundles are melted and joined together by the coalescing of the melts. In the same manner the bristle carrier is melted in a locally defined manner, namely, at the joining points, and subsequently the bristle bundles and bristle carrier are brought together. Under pressure the melts flow together and form a welded joint. It has been recognized that as a result of the melting processes, on hardening, an irregular surface would be obtained on the bristle carrier. It has therefore been proposed to smooth the rising melt by shaping and optionally also providing a depression at the weld into which can be displaced the rising melt. However, this only leads to a partial surface smoothing. It is not possible to obtain a complete smooth, flat surface, because in order to obtain an adequate melting temperature in the outer regions there is necessarily a greater heat accumulation in the central region of the bristle carrier, i.e. the molten material and the melt viscosity increases from the inside to the outside. In the case of equidirectional reworking at the weld of the individual bundles it is consequently impossible to obtain the same working result over the entire surface.
The joining of bristles and bristle carriers made from the same or different plastic materials in the molten state constitutes a type of thermal joining process. It is known (EP-A-149 996), to melt the bristle ends to form a thickened portion and with the latter still in the plastics state to shape the bristle bundles into preshaped holes on the bristle carrier. The plastic thickened portion is intended to fill the holes provided with undercuts. The irregularities on the surface due to the rising melt are inter alia to be eliminated in that the holes are initially formed without undercuts, the thickened portions are introduced into the holes and subsequently the surrounding material is shaped on to the thickened portions. For this purpose the bristle carrier is provided in the vicinity of the holes with ring-like projections, which are then shaped by plasticizing the bristle carrier material, In this case the flatness or unevenness of the bristle carrier surface is once again significantly dependent on the behavior of the melt in the holes.
With the knowledge of the disadvantages of the aforementioned process, it has been proposed (DE-A-36 37 750) to only melt the holes preshaped in the bristle carrier in the vicinity of the hole bottom and to introduce the bristle bundle, whose ends are not melted together in this case, being introduced into the hole and into the melt located at its bottom. The melt is then intended to rise in the hole and between the bristles, so as to ensure an adequate anchoring thereof. The melt volume with respect to the still free volume in the hole is selected in such a manner that the melt does not rise to the outside of the bristle carrier. Naturally this is only wishful thinking, because an adequately precise heat transfer from the heating element to the walls in the vicinity of the hole bottom For forming an always constant melting mass cannot be achieved. The bristles are substantially only non-positively or frictionally held, so that there is no extraction-resistant anchoring of the same in the bristle carrier.
In the aforementioned process (DE-A-34 03 341) the bristles within the bundle are melted with one another at the fastening-side end to form a thickened portion and the latter, preferably in the hardened state, is pressed into the locally, e.g. in the form of holes, melted bristle carrier until the melt off the latter has flowed together again behind the thickened portion due to molecular reorientation. However, it is not possible to avoid the rising of the melt and the formation of a melt bead, which subsequently has to be levelled by a smoothing or aftershaping tool or formed into a precisely defined projection. For the reasons given hereinbefore (DD-A-221 633) this process is also not completely satisfactory.